Durene process



March 5, 1957 A. P. LIEN ETAL DURENE PRocx-:ss

Filed May lO, 1955 INVENTORS: Arf/wr Lien By Dov/d A. McCall/ay MM @M5/W6 Q ATTORNEY DURENE PROCESS Arthur P. Lien, Highland, Ind., and David A. McCaulay, Chicago, lll., assignors to Standard (Bil Company, Chicago, lll., a corporation of Indiana Application May 10, 1955, Serial No. 507,412

3 Claims. (Cl. 260-668) This invention relates to the production of essentially pure durene by isomerization of isodurene, prehnitene, or mixtures of tetramethylbenzenes.

Durene, 1,2,4,5-tetramethylbenzene, has recently become of importance as a starting material for the production of chemical intermediates. The presence of two unsubstituted ring positions which are para to each other and the symmetrical positions of the methyl groups have created commercial interest in this tetramethylbenzene. The tetramethylbenzenes are available from petroleum distillates in very small amounts. By the use of HF-BFa catalyzed processes it is possible to prepare isodurene, l,2,3,5-tetramethylbenzene, in very high yield from either trimethylbenzenes or dimethylbenzenes. Unfortunately, isodurene does not have a large commercial demand at this time and it is primarily used as a high octane, high boiling component of gasoline.

An object of the invention is a process for the preperation of high purity durene. Another object is a process for the preparation of high purity durene by isomerization of the other tetramethylbenzene isomers. A particular object is a continuous process for the isomerization of a tetramethylbenze feed containing the isomers other than durene as the predominant components to yield a product consisting substantially only of durene. Other objects will become apparent in the course of the detailed description.

In the process of this invention, a tetramethylbenzene feed, which may be isodurene, prehnitene, mixtures of these two, or mixtures of durene and one or both of these other isomers, is isomerized to about the equilibrium composition with respect to durene content. Thus a mixture containing durene and one or both the other isomers must contain less than the equilibrium amount of durene. The isodurene is removed from admixture with the durene and also prehnitene by extraction with HF-BFa solvent. The durene is recovered in substantially pure form from prehnitene or other hydrocarbons which may be present after the extraction. This isodurene may be recovered from the HF-BFs solution and recycled to the isomerization Zone.

The process is described in detail in connection with the annexed drawing which forms a part of this specification. It is to be understood that the process is not limited to this particular illustrative embodiment and that many items of equipment have been omitted from the drawing, since these may be readily added by those skilled in the art.

In the drawing, feed from source 11 is passed by way of lines l2 and 13 into vessel 14. In order to eliminate side reactions as much as possible, the feed to the process contains tetramethylbenzene(s) as essentially the only aromatic hydrocarbon. The feed may be isodurene, prehnitene, or mixtures of one or both of these isomers with durene, wherein the durene content of the mixture is less than the equilibrium amount. Preferably the isodurene and/or prehnitene componentCs) predominates over the durene component. While under some conditions of opnited States Patent 2,784,242 Patented Mar. 5, 1957 rice eration the apparent equilibrium content of a dureneisodurene mixture is between about 65-70 mole percent of durene, in general the natural mixture of these two isomers appears to be about equimolar in the two isomers. The process is operable in the presence of non-aromatic hydrocarbons which do not tend to react with the tetramethylbenzenes; however, for reasons of capacity and yield, it is preferred to operate with a feed which is substantially free of non-aromatic hydrocarbons. The feed from source 11 is a natural mixture of prehnitene, isodurene and durene, wherein the prehnitene and isodurene represent about of the feed.

The isomerization reaction is catalyzed by liquid HF and BFa. In order to avoid hydrate formation and to minimize corrosion, the entire process is carried out under substantially anhydrous conditions. Commercial grade anhydrous hydrofluoric acid, which contains up to about 3% of water, may be used .in the process. In this embodiment, substantially anhydrous liquid HF from source 16 is passed by way of valved line 1'7 and line 18 into vessel 14. Sucient liquid HF is introduced to produce a distinct separate acid phase in the reaction zone. More than this amount is generally used; for example, the liquid HF usage may be between about 3 moles and as much as 50 or more moles per mole of tetramethylbenzene feed. It is preferred to use between about 6 and 12 moles of liquid HF. In this embodiment, l0 moles of commercial grade anhydrous hydrouoric acid are used per mole of tetramethylbenzene charged to vessel 14.

Boron triuoride from source 21 is passed by way of valved line 22 into vessel 14. Sufficient BFa must be used to exeit a catalytic effect and to promote the isomerization of the tetrarnethylbenzenefeed. However, as the amount of BFa is increased, the equilibrium product mixture is shifted in the direction of isodurene. In order to avoid the production of isodurene, the amount of BFa used is maintained between about 0.03 and 0.3 mole per mole of tetramethylbenzene feed. It is preferred to operate with between about 0.05 and 0.15 mole of BFa per mole of tetramethylbenzene in the feed. Y

Vessel 14 permits the intermingling of the feed, HF and BF3 and the intermingled stream is passed by way of line 23 into reactor 24. Reactor 24 is a vessel provided with a stirrer 26 driven by a motor 27 and ba'flies not numbered. Reactor 24 is provided With a trapout point 28. Any form of vessel which provides the necessary holding time at the particular temperature may be used as the isomerization reactor. (It is to be understood that sufficient pressure is maintained on the .system to keep the HF in the liquid state.) The isomerization reaction is carried out at a temperature between about 20 C. and abou-t 165 C. The preferred temperature of contacting is between about 40 and about 100 C. The tetramethylbenzene feed and the HF and BFs catalyst are vigorously responding` time. At 165 C., the necessary time of contacting may be as short as 2 or 3 minutes; on the other hand, at 20 C., the time of contacting may be as long as 20-24 hours. At the preferred temperatures of between about 40 C. and 100 C., the time of contacting needed is between about 30 minutes and 6 hours, the

longer times corresponding to the lower temperatures. In this illustration, reactor 24 is maintained at 90 C. for a time of 60 minutes. Prolonged contacting timesparticu larly at -the highertemperatures, are to be avoided because of disproportionation reactions.wherein the tetramethylbenzenes are disproportionated to pentamethylbenzene and trimethylbenzene.

yThe isomerization reactionmixture iswithdrawn from reactor 24 by way of'trapout 28 and line 31S into mixer 32. The -isomerization reaction mixture contains the HF, BF3 and a substantially equilibrium mixture of tetramethylbenzene, wherein now durene isy the predominant component, although under some conditions it may onlyl be equimolar with the isodurene as the other component. Some slight amount of trimethyl benzene and pentamethylbenzene may also be present. Mixer 32 is provided with a heat exchanger coil 33, which permits the temperature to be controlled. BFS from valved line 36 and also from source 21 and valved line 37 is introduced into mixer 32. Enough B'Fs is introduced into mixer 32 so that the 'total BF3 present is atleast equal to theisodurene present, `that is, l mole of BFa is present per mole of isodurenezpresent. When essentially pure durene` is desired, the amount of BF3 present may be somewhat in excess of 1 mole per mole of isodurene, for example, 1.05. ln this illustration, the BFg present is 1.0 mole per mole of isodurene present.

The BFs-enriched mixture is passed from mixer 32 by way of line 39 into extractor 41. The isomerization reaction continues in mixer 32 and even in extractor 41. Particularly in mixer 32 the presence of a large amount of B-Fs tends to increase the yield of isodurene. By properly adjusting the conditions of time in mixer 32 and extractor 41, the complexing of the isodurene yand the extraction of 4the durene in extractor 41 may be carried out at substantially the same temperature as that existent in reactor 24. In order to permit longer times of mixing and extraction, it is preferred to cool the BFs-enriched mixture in mixer 32and operate extractor 41 at a temperature between about 20 C. and 40 C, 'In this embodiment, extractor 41 is operated at a temperature of 25 C.

The isomerization reaction in reactor 24`is carried out in a two-phase system, since the tetramethylbenzenes vare not completely soluble in the liquid HF. The addition of the B113 in mixer 32/results in the formation of an isodurene complex with HF and BFs, which complex is completely soluble in the liquid HF. The durene and also prehnitene, if present are somewhat soluble in the acid phase because of. the solubilizing eitect `of the isodurene complex. However, under the conditions set out, 4a separate hydrocarbon phase consisting almost entirely Ot durene (and also prehnitene) will be present in addition to the acid phase which contains isodurene complex, possibly some complexed durene and physically dissolved durene (and also prehnitene). This mixture is charged by way of line 39 into extractor 41.

Extractor 41 is a vessel designed for the countercurrent contacting of two immiscible liquids. Instead of using a countercurrent tower, a series of countercurrent batch stages may also be used. 'In extractor 41, the physically dissolved durene is removed from the acid phase by contact with an inert lower boiling nonaromatic hydrocarbon. An inert hydrocarbon is necessary in order to avoid reaction, such as alkylation, with the tetramethylbenzene, and Aa lower boiling hydrocarbon is necessary in order to permit easyV distillative separation of the non-aromatic hydrocarbon and the tetramethylbenzene. Examples of suitable inert `lower' boiling hydrocarbons are butane, hexane, cyclopentane, and methylcyclopentane. `'In this embodiment, hexane is used as the wash hydrocarbon. Enough hexane is introduced from source 42 by way of valved line 43, lines 44 and 46 to remove essentially all the durene (and prehnitene) from the acid phase. -In general, between about 1 volume and 5 volumes `of hexane are usedper volume of uncomplexed tetramethylbenzenes introduced into extractor 41. In this illustration, 3.5 volumes of hexane amenace are used per volume of durene present in the mixture introduced from line 39.

lt is pointed out here that' all the tetramethylbenzenes form a complex with HF and BFS. However, the complex formed by isodurene with HF and EP3 is so much stronger than that formed by either prehnitene or durene that under the conditions where the amount of BFs amounts to not more than l mole per mole of isodurene present, the acid phase will contain essentially only isodurene as the complexed tetramethylbenzene. Because of this difference in stability, it is possible to wash out of the `acid phase the durene (and also prehnitene), leaving behind essentially pure isodurene, when only one mole ot BFa per mole of isodurene present has been used. The use of more than one mole of BF3 per mole of isodurene will result in the presence of durene and/ or prehnitene in the acid phase and in the tetramethylbenzene recovered from the acid phase.-

The hexane, ltheV extracted durene, and the separate hydrocarbon phase of durene are. removed as `a single hexane-rich stream and are introduced byv way of line 4S into vessel51',.which isprovided with internal heat exchanger 52, Vessel 51 acts as a fractionator and the lower boilinghexane is removed overhead and is recycled to extractor` 41 by way of. line 44and condensors not shown. Product durene, which is essentially free ot isodurene, and prehnitene, is removed as a bottoms product by Way of line 53. When the feed to the process results in the presence of appreciable amounts of prehnitene in the equilibrium mixture emerging from reactor 24, the bottoms product from vessel 51 will contain both durene and prehnitene. The boiling points and freezing points set out in Table I show that prehnitene and durene may be readily separated by fractional distillation owing to the 8 C. difference in boiling point. The durene from line 53 is treated-to remove traces of HF and BFg which may remain therein.

The acid phase, which is now free of dissolved tetramethylbenzenes, other than isodurene, is removed from extractor 41 by way of line 56. Since isodurene is a suitable feed to the process, the acid phase which contains isodurene, HF, BF3, andl possibly mesitylene and pentamethylbenzene may be recycled directly to vessel 14 to provide not only lrecycle isodurene, but also HF and BFs. =When operating with lower amounts of BFa, the amount of BF3 present in the acid phase may be in excess of the requirements, in which case only a portion oi the acid phase may be recycled by way of valved line S7, vessel 14 and line 23 to reactor 24.

The acid phase which is not recycled or the entire acid phase is passed by way of valvedline 58 into de composer 59, which is provided with internal heater 61. In vdecornposer '59, the HF and BFS are distillatively removed overhead by Way of valved line 62. The HF land BFS stream is passed into lseparator 63 where BF; is separated and recycled by way of line 36 to mixer 32. Liquid -HF containing dissolvedBFa is Withdrawn from separatory 63 and is recycled by way ot valved 'line 64 and line 18 to vessel 14.l

rhe decomposition of the acid phase in decomposer 59 may be accomplished either by operating at sub-atmospheric pressure or at elevated temperature and ordinary pressures. Once again it is pointed outthat'at elevated temperatures the-HF and BFa catalyze the disproportionation of isodurene to pentamethylbenzene, it the isodurene is exposedA to these` temperatures for prolonged' times.

, valved line 66. This bottoms product consists essentially of isodurene as the tetramethylbenzene and also small amounts of trimethylbenzene and pentamethylbenzene .from disproportionation reactions. The bottoms product may be sent by way of line 67 to further treatment wherein essentially pure isodurene is recovered. When it is desired to maximize the yield of durene, as is desired in this embodiment, the bottoms product is passed by way of valved line 68 and line 13 into vessel 14. In order to remove the mesitylene and pentamethylbenzene, periodically or continuously the material from line 66 is removed, fractionated and the mesitylene and pentamethylbenzene separated and the isodurene recycled.

Example 1 The ability of the HF-BFs catalyst to isomerize tetramethylbenzenes to a durene-rich mixture is demonstrated by the example below. The experiment was carried out using a 1570 ml. carbon steel reactor provided with a 1725 R. P. M. mechanical stirrer. 'Ihe tetramethylbenzene feed was free of other aromatic hydrocarbons and non-aromatic hydrocarbons. This feed had been obtained by treatment of trimethylbenzenes with HF and BFa under conditions to obtain a mixture of durene and isodurene, wherein the isodurene was by far the predominant component. The feed was introduced into the reactor; commercial grade anhydrous hydrouoric acid was added and then BFs was added to the reactor. Six moles of HF per mole of tetramethylbenzene were used and 0.1 mole of BFs per mole of tetramethylbenzene were used. The reactor and its contents were heated to 100 C. and vigorously stirred for a time of 30 minutes.

The contents of the reactor were withdrawn into a vessel filled with crushed ice which had been brought to the temperature of a Dry Ice-acetone bath. A hydrocarbon layer was decanted away from a lower aqueous acid layer. The hydrocarbons were neutralized with aqueous ammonia, water washed and dried. The dried hydrocarbons were fractionated in a column providing about 30 theoretical plates. A material corresponding to tetramethylbenzene boiling range was analyzed for physical characteristics and by infrared procedures.

Within the error of infrared technique, the tetramethylbenzene product consisted of mole percent of durene and 30 mole percent of isodurene.

Thus having described the invention what is claimed is:

1. A continuous process for the production of durene which comprises (l) isomerizing a feed consisting essentially of tetramethylbenzenes selected from the class consisting of isodurene, prehnitene and mixtures thereof with durene, wherein the component other than durene predominates, to a tetramethylbenzene mixture containing about the equilibrium amount of durene by contacting said feed, under substantially anhydrous conditions, with between about 3 and 50 moles of liquid HF pei mole of tetramethylbenzene and between about 0.05 and 0.15 mole of BFa per mole of tetramethylbenzene, at a temperature between about 40 C. and 100 C. for a time between about 30 minutes and 6 hours, the longer times corresponding to the lower temperatures, (2) withdrawing isomerization reaction mixture comprising tetramethylbenzene product, HF and BFa and adding BFs thereto in an amount sucient to have present in said mixture l mole of BFS per mole of isodurene present, (3) separating said mixture, at a temperature between about 20 C. and 40 C., in the presence of an inert, lower boiling hydrocarbon, into a hydrocarbon phase comprising said inert hydrocarbon and durene, which hydrocarbon phase is essentially free of isodurene and an acid phase comprising HF, BFs and isodurene, and (4) distillatively recovering essentially pure durene from said hydrocarbon phase.

2. The process of claim 1 wherein the acid phase is decomposed to recover tetramethylbenzenes and said tetramethylbenzenes are recycled to the isomerizing zone.

3. The process of claim 1 wherein a portion of said acid phase is recycled to the isomerizing zone.

Nightingale et al.: Journal of the American Chemical Society, vol. 63, 1941 (pages 3514-3517 relied on). 

1. A CONTINUOUS PROCESS FOR THE PRODUCTION OF DURENE WHICH COMPRISES (1) ISOMERIZING A FEED CONSISTING ESSENTIALLY OF TETRAMETHYLBENZENES SELECTED FROM THE CLASS CONSISTING ISODURENE, PREHNITENE AND MIXTURES THEREOF WITH DURENE, WHEREIN THE COMPONENT OTHERE THAN DURENE PREDOMINATES, TO A TETRAMETHYLBENZENE MIXTURE CONTAINING ABOUT THE EQUILIBRIUM AMOUNT OF DURENE BY CONTACTING SAID FEED, UNDER SUBSTANTIALLY ANHYDROUS CONDITIONS, WITH BETWEEN ABOUT 3 AND 50 MOLES OF LIQUID HF PER MOLE OF TETRAMETHYLBENZENE AND BETWEEN ABOUTG 0.05 AND 0.15 MOLE OF BF3 PER MOLE OF TETRAMETHYLBENZENE, AT A TEMPERATURE BETWEEN ABOUT 40* C. AND 100* C. FOR A TIME 